A wide variety of transistors are known. These include field effects transistors, in which the conductivity of a channel or layer of semiconductive material arranged between a source terminal and a drain terminal is controlled by application of a potential to a gate terminal. Thin-film transistors are also known. Thin film transistors (TFTs) are a type of field effect transistor (FET) typically made by depositing a thin film of a semiconductive active layer as well as a dielectric layer and metallic contacts over a supporting substrate. In certain applications, the substrate is glass, for example where the TFT is used in a liquid crystal display. TFTs can be made using a wide variety of semiconductive materials. By using transparent semiconductors and transparent electrodes, such as indium tin oxide (ITO), certain TFT devices can be made completely transparent.
In the manufacture of known TFTs, it is necessary to achieve alignment between the gate and source and drain terminals. In the past, lithographic techniques have been used to manufacture TFTs, with, typically, one mask defining the relative positions of source and drain terminals and another mask defining the positions of the gate terminals, for example where an array of a large number of TFTs is being produced. Clearly, a problem with such techniques is achieving the appropriate alignment between the finely patterned masks used at different stages of the manufacturing process. Another problem arises where the substrate has a large area and the array of TFT devices is to be distributed over that large area. Imperfections and distortions in the substrate can result in local misalignments between the source and drain and the gate of one or more TFT devices. In the manufacture of liquid crystal displays, for example, these misalignments can result in faulty pixels, which degrade the display and indeed may render it unacceptable.
Additional problems associated with the manufacture of thin-film transistors include the following. The minimum size of the device features is limited by the manufacturing techniques, which in turn limits the density of devices which can be formed on a single substrate, for example limiting the resolution of a display incorporating an array of TFTs. The correct operation of the thin film transistors is typically highly sensitive to the quality of the interfaces between the different device layers, such as the interface between the substrate and semiconductor material, the interface between the semiconductor material and the gate dielectric, and the interface between the gate dielectric and the gate electrode itself. These interfaces are degraded, and the performance of the TFT itself is degraded, when there are imperfections in these interfaces, such as when these interfaces become contaminated during the manufacturing process. It is therefore desirable to develop a method of manufacturing thin-film transistors which assists in the formation of clean, defect free interfaces.